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Transcript
Eukaryotes: The Other Guys Part I: Eukaryotic Microbial Structure and Function • • • • Eukaryotic Microorganisms prominent members of ecosystems useful as model systems and industry some are major human pathogens two groups – protists – fungi Figure 4.1 • • • • Common Features of Eukaryotic Cells membrane-bound nuclei membrane-bound organelles that perform specific functions intracytoplasmic membrane complex serves as transport system more structurally complex and generally larger than bacterial or archaeal cells Figure 4.2 Figure 4.3 Table 4.1 • • • • • Eukaryotic Cell Envelopes consists of the plasma membrane and all coverings external to it plasma membrane is a lipid bilayer – major membrane lipids include phosphoglycerides, sphingolipids, and cholesterol, all of which contribute to strength of membrane unlike the peptidoglycan in the cell wall of Bacteria and Archaea, many eukaryotes lack or have a chemically distinct cell wall cell walls of photosynthetic algae have cellulose, pectin, and silica cell walls of fungi consist of cellulose, chitin, or glucan Figure 4.4 The Cytoplasm of Eukaryotes • consists of liquid, the cytosol, and many organelles • cytoskeleton – vast network of interconnected filaments within the cytoplasmic matrix – filaments that form the cytoskeleton: microfilaments (actin), microtubules, intermediate filaments, and motor proteins – plays role in both cell shape and cell movement Figure 4.5 • • • • Microfilaments minute protein filaments, 4 to 7 nm in diameter scattered within cytoplasmic matrix or organized into networks and parallel arrays composed of actin protein involved in cell motion and shape changes Figure 4.6 Intermediate Filaments • heterogeneous elements of the cytoskeleton, ~10 nm in diameter • keratin and vimentin classes • role in cell is unclear – play structural role – some shown to form nuclear lamina – others help link cells together to form tissues • • • • • • • • Microtubules shaped like thin cylinders ~25 nm in diameter of tubulin dimers help maintain cell shape involved with microfilaments in cell movements participate in intracellular transport processes Secretory Endocytic Pathway intricate complex of membranous organelles and vesicles that move materials into the cell from outside, from inside to outside, and within the cell Endoplasmic reticulum (ER) Golgi apparatus lysosomes Endoplasmic Reticulum (ER) • irregular network of branching and fusing membranous tubules and flattened sacs (cisternae – s., cisterna) • rough ER – ribosomes attached – synthesis of secreted proteins by ER-associated ribosomes • smooth ER – devoid of ribosomes – synthesis of lipids by ER-associated enzymes • • Functions of ER transports proteins, lipids, and other materials within cell major site of cell membrane synthesis The Golgi Apparatus • membranous organelle made of cisternae stacked on each other • cis and trans faces • dictyosomes – stacks of cisternae • involved in modification, packaging, and secretion of materials • • • • Lysosomes membrane-bound vesicles found in most eukaryotes involved in intracellular digestion contain hydrolases, enzymes which hydrolyze molecules and function best under slightly acidic conditions maintain an acidic environment by pumping protons into their interior The Secretory Pathway • used to move materials to various sites within the cell, as well as to either the plasma membrane or cell exterior • proteins destined for the cell membrane, endosomes, and lysosomes or secretion are synthesized by ribosomes on rough endoplasmic reticulum (RER) • targeted to RER lumen and are released in small budding vesicles from RER • • released in small vesicles cis face of Golgi apparatus trans face of Golgi apparatus – modification of proteins occurs in Golgi; targets protein for final destination transport vesicles released from trans face of Golgi • • after release some vesicles deliver their contents to endosomes and lysosomes two types of vesicles deliver proteins to cell membrane – constitutive delivery to membrane – Secretory vesicles in multicellular eukaryotes store proteins until signal to release • quality assurance mechanism – unfolded or misfolded proteins are secreted into cytosol, targeted for destruction by ubiquitin polypeptides – proteasomes destroy targeted proteins Figure 4.9 The Endocytic Pathway • endocytosis – used to bring materials into the cell – solutes or particles taken up and enclosed in vesicles pinched from plasma membrane – in most cases materials are then delivered to lysosome and destroyed Types of Endocytosis • phagocytosis use of cell surface protrusions to surround and engulf particles fuse with lysosomes and resulting vesicles called phagosomes • clathrin-dependent endocytosis – involves membrane regions coated on cytoplasmic side with the protein clathrin (coated pits) – coated pits have external receptors that specifically bind macromolecules – pinching off of coated pits forms coated vesicles – called receptor-mediated endocytosis – – Pinocytosis • In the process of pinocytosis, the plasma membrane forms an invagination.What ever substance is found within the area of invagination is brought into the cell. • In general, this material will be dissolved in water and thus this process is also referred to as "cellular drinking" to indicate that liquids and material dissolved in liquids are ingested by the cell. • This is opposed to the ingestion of large particulate material like bacteria or other cells or cell debris. Figure 4.10 • • • • Autophagy delivery of materials to be digested by route that does not involve endocytosis macroautophagy involves digestion and recycling of cytoplasmic components double membrane surrounds cell component forming an autophagosome autophagosome fuses with a lysosome Once Lysosome Is Formed… • digestion occurs without release of lysosome enzymes into cytoplasmic matrix • as contents are digested, products leave lysosome and can be used as nutrients • resulting lysosome called a residual body which can release contents to cell exterior by process called lysosome secretion Organelles Involved in Genetic Control of the Cell • • Nucleus Ribosomes The Nucleus • membrane-bound spherical structure that houses genetic material of eukaryotic cell • contains dense fibrous material called chromatin – complex of DNA, histones, and other proteins – five types of histones form nucleosomes • H1, H2A, H2B, H3, and H4 – chromatin condenses into chromosomes during division Figure 4.11 • • • • • • • nuclear envelope – double membrane structure that delimits nucleus – continuous with ER – penetrated by nuclear pores • associated proteins make up the nuclear pore complex • pores allow materials to be transported into or out of nucleus Figure 4.12 The Nucleolus ≥ 1 nucleolus/nucleus organelle but not membrane enclosed important in ribosome synthesis – directs synthesis and processing of rRNA – directs assembly of rRNA and ribosomal proteins to form partially completed ribosomal subunits – ribosomes mature in cytoplasm Figure 4.13 Eukaryotic Ribosomes larger than the 70S bacterial and archaeal ribosomes 80S in size – 60S + 40S subunits may be attached to ER or free in cytoplasmic matrix Figure 4.14 • proteins made on ribosomes of RER are often secreted or inserted into ER membrane as integral membrane proteins • free ribosomes synthesize nonsecretory and nonmembrane proteins – some proteins are inserted into organelles Organelles Involved in Energy Conservation • Mitochondria • Hydrogenosomes • Chloroplasts Figure 4.15 • • • • • Mitochondria “the powerhouses of the cell” are found in most eukaryotic cells site of tricarboxylic acid cycle activity site where ATP is generated by electron transport and oxidative phosphorylation about the same size as bacterial cells reproduce by binary fission as do bacterial cells Mitochondrial Structure • • • outer membrane – contains porins similar to the outer membrane of gram-negative bacteria inner membrane – highly folded to form cristae (s., crista) – location of enzymes and electron carriers for electron transport and oxidative phosphorylation matrix enclosed by inner membrane – contains ribosomes (same size as bacterial), mitochondrial DNA (may be closed circular like bacterial DNA), and large calcium phosphate granules – contains enzymes of the tricarboxylic acid cycle and enzymes involved in catabolism of fatty acids Figure 4.16 Hydrogenosomes • small energy conservation organelles in some anaerobic protists • descended from common mitochondrial ancestor – double membrane, no cristae, usually lack DNA – ATP is generated by fermentation process rather than respiration – CO2, H2, and acetate are products Figure 4.17 Chloroplasts • type of plastid – pigment-containing organelles observed in plants and algae • site of photosynthetic reactions • surrounded by double membrane • Chloroplast Structure the stroma (a matrix) is within inner membrane – contains DNA, ribosomes, lipid droplets, starch granules, and thylakoids • thylakoids – flattened, membrane-delimited sacs – – • • grana (s., granum) – stacks of thylakoids site of light reactions (trapping of light energy to generate ATP, NADPH, and oxygen) stroma is site of dark reactions of photosynthesis (formation of carbohydrates from water and carbon dioxide) algal chloroplasts many contain a pyrenoid – participates in polysaccharide synthesis Figure 4.18 External Cell Covering • • Cilia Flagella Cilia and Flagella • • cilia (s., cilium) – 5-20 µm long – beat with two phases, working like oars flagella (s., flagellum) – 100-200 µm long – move in undulating fashion • tinsel – tip pulls cell along • whiplash – naked flagellum Figure 4.19 Figure 4.20 Ultrastructure of Flagella and Cilia • membrane-bound cylinders ~2 µm in diameter • axoneme – set of microtubules in a 9 + 2 arrangement • basal body – at base of flagellum or cilium – directs synthesis of flagella and cilia Figure 4.22 Comparison of Bacterial, Archaeal, and Eukaryotic Cells • differences in eukaryotic cells – eukaryotic nucleus – larger, more complex – meiosis, mitosis – complex processes • molecular unity basic to all three cells – biochemical processes, metabolic pathways – genetic code Table 4.2 Figure 4.23